To help address the backflow issue you're encountering, a few aspects should be considered:

• Flow Control Method: How are you introducing the liquid into the microfluidic channel? Are you using a pressure controller or a syringe pump? This information is important because different methods can influence the flow behavior, including the potential for backflow. (check differences here: https://www.elveflow.com/elevate-your-microfluidic-precision-with-our-pressure-controllers/?utm_source=researchgate.com&utm_medium=social&utm_campaign=rg_backlow)
• Flow Resistance Considerations: In general, to minimize or avoid backflow in microfluidic systems, it's crucial to ensure that the fluidic resistance upstream of the junction (in your case, the two branches of the T) is higher than the resistance downstream. This differential helps direct the flow more effectively and prevents fluid from moving backward.

By adjusting the channel dimensions or applying this principle, you may be able to mitigate the backflow you're experiencing.

To address backflow issues in your PDMS microfluidic T-channel chip, follow these steps:

Ensure a Flawless Fabrication Process: First, ensure that your chip’s fabrication process is flawless and that no blockages are introduced during the process. Even minor defects can obstruct flow, leading to backflow or irregular fluid behavior. After fabrication, perform an airflow test by passing air through the channels to check for any obstructions or irregularities. This will help confirm that the channels are open and smooth. Follow up the airflow test with a liquid flow test using a colored solution. This allows you to visually confirm the uniform flow through the channels and quickly identify any potential backflow areas.

Sample Preparation: Ensure your sample is free from particulates that could clog the microchannels by filtering it through a 0.22 µm filter. Even small particles can accumulate and cause blockages or flow issues, so filtration is a key preventive measure. It would help if you also considered the viscosity of your sample, as it influences the flow of liquids.

Flow Rate Control: I don't know how many phase chips you are using but generally you should understand the flow rate ratios of the different phases and make sure the outer phase flow rate is higher than the inner phase flow rates. I strongly suggest you use a syringe pump or a flow controller to introduce your sample into the channels. These tools provide precise control over the flow rate, which is critical in preventing backflow. If backflow persists, try incrementally increasing the flow rate. Higher flow rates can sometimes overcome the resistance that leads to backflow. However, this should be done cautiously to avoid over-pressurizing the system.

Gas-Assisted Flow Considerations: If your setup involves gas-assisted flow, backflow is common when the gas pressure is too high relative to the liquid pressure. The gas can displace the liquid, causing it to move in the reverse direction. Balancing Gas and Liquid Pressure: Ensure that the liquid flow rate is equal to or greater than the gas flow rate to prevent this imbalance. If the gas pressure is too high, it will force the liquid backward. Adjust both flows to maintain equilibrium, which should help reduce backflow.

By following these steps, you should be able to mitigate backflow in your PDMS microfluidic chip effectively. Additionally, always monitor the flow rates and pressures in real time to adjust them as necessary during experiments.

By the way, you should also filter the colored liquid you use in the testing process. Hope this will help you solve your problem